In the initial phase of infection, the innate immune system generates a rapid and potent inflammatory response. This response aims at blocking dissemination of the infectious agent, with subsequent activation of T cells and B cells that provide the acquired immune response against the pathogen. Recognition of pathogen-related components by immune cells occurs through pathogen recognition receptors (PRRs). PRRs are present on cell surfaces, in endosomes, and in the cytosol. Toll-like receptors (TLRs) represent an important family of PRRs (see, e.g., Gay and Gangloff (2007) Ann. Rev. Biochem. 76, 141-1659). TLRs are expressed on various subsets of immune cells such as dendritic cells (DCs), monocytes and macrophages. DCs and macrophages as well as monocytes are professional antigen-presenting cells that play an important role in the induction and maintenance of innate and adaptive immune responses. Due to their functional properties and prominent expression of TLRs, DCs represent promising candidates for TLR agonist-based vaccination strategies against tumors and pathogens (reviewed in, e.g., Palucka and Banchereau (2012) Nature Reviews Cancer 12 (4), 265-277).
Polyinosinic:polycytidylic acid (poly(I:C)) is a potent activator of innate immunity (Field et al. (1967) Proc. Natl. Acad. Sci. U.S.A., 58, 1004-1010) and a potent inducer of apoptosis in cancer or tumor cells (melanoma: Weber et al. (2010) Cell Death Differ. 17(6), 942-951; prostate cancer: Paone et al. (2008) Carcinogenesis 29(7), 1334-1342; breast cancer: Salaun et al. (2006) J. Immunol. 176(8), 4894-4901; hepatocellular carcinoma: Guo et al. (2012) Oncology Reports 27(2), 396-402; lung cancer: Estomes et al. (2012) Cell Death Differ. 19(9), 1482-1492; neuroblastoma: Chuang et al. (2011) J. Biomed Sci. 18, 65; Van et al. (2012) FASEB J. 26(8), 3188-3198). Poly(I:C) activates DCs through combined targeting of various innate immunity pathways including TLR3. Major disadvantages of poly(I:C) comprise its undefined chemical structure and very poor homogeneity which is due to its manufacturing process (Grunberg-Manago et al. (1955) Science 122 (3176), 907-910). Poly(I:C) is composed of a mixture of single-stranded and double-stranded RNAs. This is mainly due to limited solubility and difficult reconstitution of poly(I:C) that requires heating (50 to 60° C.) and slow cooling over many hours to achieve reannealing of the poly(I) and poly(C) strands. As a consequence, poly(I:C) has a reported toxicity in clinical trials, ranging from hypersensitivity to coagulopathy, renal failure, or systemic cardio-vascular failure (Robinson et al. (1976) J. Nat. Cancer Inst. 57 (3), 599-602). A further frequent problem of double-stranded RNA (dsRNA) compounds such as poly(I:C) is their rapid degradation in body fluids by RNAses, with a reported half-life of a few minutes (Levy et al. (1975) J. Inf. Dis., 132, p. 434; Bumcrot et al. (2006) Nat. Chem. Biol. 2 (12), 711-719) and subsequent unpredictable pharmacokinetics of degradation products. Optimization of physicochemical properties of poly(I:C) has led to the generation of derivatives that have increased stability in body fluids (such as polylCLC; see Levy et al. 1975, supra), or reduced toxicity through reduced stability in body fluids (such as poly(I:C12U; see, e.g., Basani et al. (2009) Vaccine 27 (25), 3401-3404).
WO-A-2013/064584 discloses TLR3 agonists which are ribonucleic acids comprising at least one segment of double-stranded structure of at least 45 bp wherein said at least one segment of double-stranded structure has a first and a second end each having at least 3 to 10 G/C bp within the last 6 to 20, respectively, bp calculated from the last bp of the respective end of the double-stranded structure, and wherein the nucleotide sequence between the last 6 to 20 bp at each end is heteropolymeric.
Naumann et al. (2013) Clinical and Developmental Immunology, available under the URL http://www.hindawi.com/journals/cdi/2013/283649/, describe the activation of DCs through the TLR3 agonist (G:C)100 named “RGC100”.